Typically, in a hydrodynamic bearing, a shaft rotatably contained within a housing is surrounded by a series of foils cantilevered from the housing which wrap the shaft in a series of line contacts parallel to the axis of the shaft under a predetermined preload or tension. This wrapping creates about the surface of the shaft a series of wedge shaped pockets. Relative high speed rotation of the shaft relative to the pockets develops or induces a viscous shear which wipes or draws a fluid, such as ambient air, between the shaft surface and the foils to create a very low friction, supporting gas film. The preload affects significantly the stiffness of the bearing and its operating characteristics, such as start-up torque.
While such a bearing works well at high speeds, problems exist during startup and coast down, when the induced viscous shear support film is absent. At startup, the shaft rests and drags on the bottom of the housing, creating a large starting torque, and a large eccentricity between the starting and final position of the center axis of the rotating shaft. This eccentricity creates a problem in certain applications, such as turbo chargers, since such shaft misalignment decreases efficiency. Other unique problems are present in the turbo charger environment, including thermal expansion of the housing to which the foils are mounted, which affects the preload of the foils, and foil wear from the frequent starts and stops, which also affects the tension or preload.
The prior art discloses some foil bearings in which the foil tension is adjustable. Gross et al U.S. Pat. No. 3,506,314 shows a foil bearing in a turbine application wherein a shaft is cradled within a housing by several foils in the form of slings, hung at one end to the inside of the housing, with the other end hung from a manually operable adjustable nut. This allows the slings to be shortened or tightened against the shaft. Marley U.S. Pat. No. 3,434,761 shows, in one embodiment, a foil bearing in a turbo expander application wherein cantilevered wrapping foils are spring loaded against the shaft surface. Adjusting screws threaded into the housing and engageable with the springs are manually adjusted to increase or decrease the tension of the foils against the shaft. The disadvantages of the systems are that a quick and simple simultaneous tension adjustment of the foils is not possible, each foil must be separately and manually adjusted. Nor may the foils be simultaneously and automatically adjusted in response to various bearing parameters, such as shaft angular rotation speed or temperature effects.